Field of the Invention
The present invention relates to carbon materials comprising carbon nanotubes, powders comprising carbon nanotubes, and methods of making carbon nanotubes. The present invention has particular, but not exclusive, application to the manufacture of components such as electrical conductors. Suitable electrical conductors include wires (e.g. for electrical motors) and cables (e.g. for transmitting electrical power).
Related Art
Carbon nanotubes are allotropes of carbon, which are tubular and typically have a diameter in the nanometre range. The carbon atoms of a carbon nanotube are each covalently bonded to three other carbon atoms, to create a “hexagonal” lattice which forms a wall of the tube. Accordingly, a carbon nanotube can be thought of as a “rolled” graphene sheet. Single-walled carbon nanotubes have a single layer of carbon atoms. Double- and multi-walled carbon nanotubes have two or more layers of carbon atoms, respectively.
The chirality of carbon nanotubes can vary, depending on the orientation of the hexagonal lattice of the notional graphene sheet with respect to the tube axis. Carbon nanotube chirality will be well understood by a person skilled in the art. For example, carbon nanotubes may have armchair chirality or zigzag chirality. Carbon nanotubes with a chirality intermediate an armchair and a zigzag chirality are generally referred to as chiral carbon nanotubes.
Without wishing to be bound by theory, it is believed that all single-walled armchair carbon nanotubes are electrically conductive, regardless of their diameter (i.e. it is believed that all single-walled armchair carbon nanotubes are metallic). Zigzag and chiral carbon nanotubes may be metallic or semiconducting.
(Nanotube chirality and properties, such as metallic and semiconducting properties, are explained in detail in Reference 4, which is incorporated herein by reference in its entirety.)
Production of bulk carbon nanotube materials is of particular interest. Such carbon nanotube materials can have particularly beneficial properties, such as relatively low density and high strength.
WO2008/132467 describes densifying carbon nanotubes to improve the efficiency of carbon nanotube packing, in order to provide a fibre or film. For example, a density enhancement agent such as divinyl benzene may be applied to the carbon nanotubes, in order to improve the packing of the carbon nanotubes, which provides a higher strength material. The fibres and films described in WO2008/132467 may be at least one metre long.
Similarly, Koziol et al1 have described the production of carbon nanotube fibres with high specific strength and high specific stiffness. This document describes the production of carbon nanotubes by thermal chemical vapour deposition (CVD). In the methods described, the resulting “aerogel” of carbon nanotubes is drawn into a fibre, which is run through an acetone vapour stream to enhance the densification. A winding rate of up to 20 m min−1 is employed to draw the fibre.
Motta et al2 have described the effect of sulphur as a promoter of carbon nanotube formation. They describe using thiophene as a sulphur precursor in iron catalysed thermal CVD, to produce nanotubes with a diameter of between 4 nm and 10 nm, which were typically double-walled. The iron catalyst particles were about 5 nm to 10 nm. The resulting aerogel was drawn into a fibre, with a winding rate of 20 m min−1. Motta et al report a high carbon nanotube growth rate of up to 0.1 to 1 mm sec−1.
The carbon materials produced by the methods described in these documents include a mixture of carbon nanotubes with a wide distribution of diameters and with a wide distribution of chiralities (including armchair, zigzag and intermediate chiralities). Increasing the degree of control of carbon nanotube formation would provide a greater control of the properties of the resulting carbon materials produced, but although many researchers have made efforts to provide such control, the present inventors are not aware of any disclosure of significant recent progress in this area.